refactor: extract shared merge_model_metrics helper (#47)

[microsasa]

Closes #47

Problem

ModelMetrics dict merging was implemented twice with divergent approaches:

• parser.py (build_session_summary): Creates new ModelMetrics/RequestMetrics/TokenUsage objects on every merge — safe but verbose.
• report.py (_aggregate_model_metrics): Uses model_copy() on first insertion, then mutates the copy in-place — fragile if copy is ever removed.

Any future bug fix or field addition (e.g. a new token type on TokenUsage) had to be applied in both places.

Solution

Extracted a shared merge_model_metrics(base, additional) helper in models.py that:

• Returns a new dict without mutating either input
• Deep-copies base entries and accumulates additional entries by model name
• Is the single source of truth for merging model metrics

Both parser.py and report.py now delegate to this helper.

Changes

File	Change
src/copilot_usage/models.py	Added merge_model_metrics() function
src/copilot_usage/parser.py	Replaced 20-line inline merge loop with merge_model_metrics() call
src/copilot_usage/report.py	Replaced _aggregate_model_metrics body with merge_model_metrics() call
tests/copilot_usage/test_models.py	Added TestMergeModelMetrics with 7 test cases

Tests

New TestMergeModelMetrics class covers:

• Both dicts empty
• Empty base / empty additional
• Overlapping keys — all 6 fields accumulate correctly
• Disjoint keys kept separate
• No mutation of base input
• No mutation of additional input

Existing test_parser.py and test_report.py tests (including TestAggregateModelMetrics) remain unchanged and validate regression.

Generated by Issue Implementer · ◷

Warning

⚠️ Firewall blocked 4 domains

The following domains were blocked by the firewall during workflow execution:

• astral.sh
• conda.anaconda.org
• pypi.org
• repo.anaconda.com

To allow these domains, add them to the network.allowed list in your workflow frontmatter:

network:
  allowed:
    - defaults
    - "astral.sh"
    - "conda.anaconda.org"
    - "pypi.org"
    - "repo.anaconda.com"

See Network Configuration for more information.

[Copilot]

Pull request overview

This PR refactors duplicated ModelMetrics merging logic into a shared merge_model_metrics(base, additional) helper to ensure consistent, non-mutating aggregation across the codebase.

Changes:

• Added merge_model_metrics() helper to centralize model-metrics merging behavior.
• Updated parser.py and report.py to delegate merging to the shared helper.
• Added dedicated unit tests for merge_model_metrics().

Reviewed changes

Copilot reviewed 4 out of 4 changed files in this pull request and generated 3 comments.

File	Description
src/copilot_usage/models.py	Introduces merge_model_metrics() helper for non-mutating metric aggregation.
src/copilot_usage/parser.py	Replaces inline shutdown-cycle metric merge loop with merge_model_metrics().
src/copilot_usage/report.py	Replaces _aggregate_model_metrics loop body with merge_model_metrics().
tests/copilot_usage/test_models.py	Adds TestMergeModelMetrics coverage for empty/overlap/disjoint/non-mutation cases.

---

💡 Add Copilot custom instructions for smarter, more guided reviews. Learn how to get started.